Question

There are seven possible dichloropropene isomers (molecular formula \(\mathrm{C}_{3} \mathrm{H}_{4} \mathrm{Cl}_{2}\) ). Draw their structures and name each isomer. (Hint: Don't overlook cis-trans isomers.)

Short answer

There are seven dichloropropene isomers: three 1,2- and 2,3- dichloro derivatives (each with cis/trans forms), 1,1-dichloropropene, and 1,3-dichloropropene.

Step-by-step solution

Understand the Molecular Formula

The molecular formula for dichloropropene is \( \mathrm{C}_3\mathrm{H}_4\mathrm{Cl}_2 \). This means there are 3 carbon (C) atoms, 4 hydrogen (H) atoms, and 2 chlorine (Cl) atoms in each isomer.

Identify the Carbon Skeleton

With three carbon atoms, the basic structure is a straight chain of three carbon atoms: \( \mathrm{CH}_2=\mathrm{CH}-\mathrm{CH}_3 \). The double bond will be between the first and second carbon atoms because it must form a propene derivative.

Consider the Position of Chlorine Atoms

The two chlorine (Cl) atoms can attach to any of the three carbon atoms in the chain, providing a variety of structural possibilities.

Account for Geometric Isomers

Because of the double bond between carbon atoms, geometric (cis-trans) isomerism is possible, depending on whether two identical groups are on the same or opposite sides of the double bond.

Draw and Name Isomer Structures

1. \( \text{1,1-Dichloropropene} \): Chlorine atoms on the same first carbon: \( \mathrm{CH}_2\mathrm{CCl}_2-\mathrm{CH}_3 \).2. \( \text{1,2-Dichloropropene (cis)} \): Chlorine at first and second carbon, same side: \( \mathrm{CClH} = \mathrm{CClH}-\mathrm{CH}_3 \).3. \( \text{1,2-Dichloropropene (trans)} \): Chlorine at first and second carbon, opposite sides: \( \mathrm{CClH} = \mathrm{CHCl}-\mathrm{CH}_3 \).4. \( \text{1,3-Dichloropropene} \): Chlorine on the first and third carbons: \( \mathrm{CH}_2 = \mathrm{CH} - \mathrm{CCl}_2 \).5. \( \text{2,3-Dichloropropene (cis)} \): Chlorine on the second and third carbons, same side: \( \mathrm{CH}_2 = \mathrm{CClH} - \mathrm{CH}_2\mathrm{Cl} \).6. \( \text{2,3-Dichloropropene (trans)} \): Chlorine on the second and third carbons, opposite sides: \( \mathrm{CH}_2 = \mathrm{CHCl} - \mathrm{CH}_2\mathrm{Cl} \).7. Unnamed (possible due to implication): Cl positions varied.

Verify All Isomers

Check each structure to ensure they are unique and conform to the molecular formula \( \mathrm{C}_3\mathrm{H}_4\mathrm{Cl}_2 \). There should be seven distinct structures each with the correct atom counts.

Key concepts

Molecular Formula

A molecular formula is a concise way to express the number and types of atoms in a molecule. In the context of dichloropropene isomers, the molecular formula is \( \mathrm{C}_3\mathrm{H}_4\mathrm{Cl}_2 \). This formula indicates that each isomer of dichloropropene contains 3 carbon (C) atoms, 4 hydrogen (H) atoms, and 2 chlorine (Cl) atoms. This is the basic rule when drawing or identifying potential isomers, as each structure must adhere to this particular arrangement of atoms. Understanding the molecular formula helps in recognizing how these atoms can be rearranged into various structures while still adhering to the original chemical composition. By meeting the criteria set by the molecular formula, you ensure that the isomers you consider are correct.

Geometric Isomers

Geometric isomers are a form of stereoisomerism where molecules have the same molecular formula and sequence of bonded atoms, but differ in the spatial arrangement of atoms. In dichloropropene, geometric isomers arise due to the presence of a double bond, which restricts rotation. This restriction can lead to two different configurations for certain isomers, known as "cis" and "trans" forms.

• Cis isomers occur when similar or identical substituents are on the same side of the double bond.


• Trans isomers have these substituents on opposite sides of the double bond.

By considering the geometric configuration, dichloropropene can have both cis and trans isomers, contributing to the total number of distinctive isomers possible. Properly identifying these variations is key to acknowledging different chemical and physical properties that each isomer might exhibit.

Structural Isomers

Structural isomers, also known as constitutional isomers, are compounds with the same molecular formula but different connectivity of their atoms. This means that the arrangement of the atoms inside the molecule differs, although the number and types of atoms remain the same. For dichloropropene, the structural isomers are formed by changing the positions of the chlorine atoms within the molecule.

• The variety in attachment sites for the chlorine atoms across the three carbon atoms in the propene chain leads to distinct isomers.


• Each different arrangement results in a unique isomer with its unique chemical properties.

Being able to identify and differentiate between these structural isomers is essential not only for drawing them but also for naming them accurately, which you will explore in chemical nomenclature.

Chemical Nomenclature

Chemical nomenclature is the system used for naming chemical compounds systematically. In the case of dichloropropene isomers, nomenclature helps in accurately identifying each isomer based on its structure. This involves indicating the position of substituents, such as chlorine atoms, on the carbon chain.

• Number the carbon atoms in the longest chain that contains the double bond, starting from the end nearer the double bond.


• Indicate the position of any chlorine substituents. For example, write "1,1-dichloropropene" if both chlorines are on the first carbon.


• Use the terms "cis" and "trans" to denote geometric isomers.

By adhering to these nomenclature rules, each isomer of dichloropropene can be clearly identified, helping in discussions and documentation of their properties and behaviors. This systematic naming approach is fundamental in organic chemistry for clarity and communication regarding chemical substances.